Fe-N-C Catalysts Synthesized by Non-Contact Pyrolysis of Gas Phase Iron

ABSTRACT

Me-N—C catalysts, wherein Me can include a transition metal, Mn, Fe, Co, or a combination of metals with Me-INU moieties located at the exterior surface of the Me-N—C catalysts are produced by a chemical vapor deposition synthesis. The synthesis methods can utilize non-solid-contact pyrolysis wherein a metal salt can be vaporized. Gaseous metal from the vaporized metal salt can displace a metal M from the N—C zeolitic imidazolate framework. The non-solid-contact pyrolysis does not mix solid iron precursors (e.g., Me=Mn, Fe, or Co) with the solid N—C zeolitic imidazolate framework precursors during or before the synthesis, which improves the process compared to conventional methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/945,861, filed 9 Dec. 2019, which is hereby incorporated by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant NumberDE-EE0008416 awarded by the Department of Energy. The government hascertain rights in the invention.

BACKGROUND

Electric vehicles and hybrid electric vehicles are increasinglyimportant for reducing carbon emissions. The global electric vehiclemarket size was valued at $118,864.5 million in 2017. The market isexpected to increase almost five-fold to reach an estimated globalmarket size of 567 billion U.S. dollars by 2025. There is growing use ofrenewable fuels like biodiesel, bioethanol, and methanol for internalcombustion engines, but these generate concerns over emissions ofparticle pollutants, carbon dioxide, and other gases. Fuel cell vehiclesor fuel cell electric vehicles utilize a fuel cell in place of a batteryor in combination with a battery or supercapacitor. The fuel cell cangenerate electricity to power the vehicle motor, typically using oxygenfrom the air and compressed hydrogen, for long distances. Most fuel cellvehicles emit only water and heat and are classified as zero-emissionsvehicles. Hydrogen fuel cell vehicles based on proton exchange membranefuel cells (PEMFCs) were first commercialized in 2014.

Catalysts used for the oxygen reduction reaction (ORR) in PEMFCscurrently are Pt-alloys. The cost of automotive fuel cell systems isstill prohibitively high, due primarily to the high loading of Pt on thePEMFC cathode. Thus, there is a need to replace Pt-alloys in suchcatalysts with earth-abundant, inexpensive materials, i.e., platinumgroup metal (PGM)-free materials. The leading PGM-free catalysts for theORR in PEMFCs are single transition metal (Me) atoms, wherein Me=Mn, Fe,or Co embedded in a nitrogen-doped carbon matrix (Me-N—C). However, theORR activities of such Me-N—C catalysts up to now have been inferior tothat of Pt. The Department of Energy has set technical targets forimproved future performance of PGM-free catalysts and other componentsof PEMFCs (Thompson, S T; Wilson, A R, et al., 2018). Thus, there is aneed for Me-N—C catalysts with improved activity.

SUMMARY

The present technology provides chemical vapor deposition methods tosynthesize Me-N—C catalysts (wherein Me=Mn, Fe, Co, or a combination ofmetals) utilizing non-solid-contact pyrolysis. For example, an ironprecursor, such as anhydrous FeCl₃, and an N—C zeolitic imidazolateframework (N—C or N—C zeolitic imidazolate framework) can be placed intotwo separate containers for pyrolysis, rather than mixed during orbefore the pyrolysis. The FeCl₃ can have a boiling point at about 316°C. Thus, the FeCl₃ can be readily evaporated at a temperature of about750° C. After contacting the N—C zeolitic imidazolate frameworksubstrate, the gas phase FeCl₃ incorporates into the N—C zeoliticimidazolate framework substrate, forming dense Fe—N₄ sites. Theresulting Fe—N—C catalyst exhibits the highest known ORR activity whenused in a H₂—O₂ PEMFC at 0.9 V (FIG. 4 ). Multiple characterizations(such as inductively coupled plasma, Table 2) show that the highactivity can be ascribed to the ultrahigh density of Fe—N₄ sites. TheN—C zeolitic imidazolate framework can be optimized for the chemicalvapor deposition method. Gaseous FeCl₃ can be directed towards the N—Czeolitic imidazolate framework. The Me-N—C catalysts disclosed hereincan serve as the cathode catalysts in PEMFCs in commercial electricvehicles and in other applications.

Catalysts and cathodes and methods of making catalysts and cathodesdisclosed herein can be provided in a range of useful configurations andforms as known in the art of fuel cells, catalysts, electrochemistry,and batteries. For example, the catalyst materials can be deposited ascatalyst materials on a cathode.

The present technology can be further summarized by the following listof features.

1. An Me-N—C catalyst comprising Me atoms;

wherein at least about 90% of the Me atoms in the Me-N—C catalyst are inMe-N₄ moieties; and

wherein a ratio of the Me-N₄ moieties located at an exterior surface ofthe Me-N—C catalyst to the Me-N₄ moieties located within the Me-N—Ccatalyst is in the range from about 90:1 to about 100:1.

2. The Me-N—C catalyst of feature 1, wherein Me is Fe, Mn, Co, or acombination thereof.3. The Me-N—C catalyst of any one of the preceding features, wherein theMe-N—C catalyst comprises N—C sites; and wherein at least about 99% ofthe N—C sites at the exterior surface of the Me-N—C catalyst are boundto Me.4. The Me-N—C catalyst of any one of the preceding features, wherein atleast about 90% of the Me-N₄ moieties are accessible by a gas-phasecontacting the catalyst.5. The Me-N—C catalyst of any one of the preceding features, wherein atleast about 99% of the Me-N₄ moieties are accessible by a gas-phasecontacting the catalyst.6. The Me-N—C catalyst of any one of the preceding features, wherein Meis Fe, and the Fe—N—C catalyst comprises not less than about 2 weight %of Fe relative to the total weight of the Me-N—C catalyst.7. The Me-N—C catalyst of any one of the preceding features, wherein Meis Fe, and wherein the Brunauer-Emmett-Teller area of the Fe—N—Ccatalyst is at least about 1500 m²·g⁻¹.8. The Me-N—C catalyst of any one of the preceding features, wherein Meis Fe, and wherein the electrochemical surface area of the Fe—N—Ccatalyst is at least about 1800 m²/g.9. The Me-N—C catalyst of any one of the preceding features, wherein thecatalyst is capable of providing an IR-corrected current ≥0.033 mA·cm⁻²at 0.90 V in an H₂—O₂ proton exchange membrane fuel cell at about 1.0bar and 80° C.10. A cathode for a fuel cell comprising the Me-N—C catalyst of any oneof the preceding features.11. The cathode of feature 10, wherein the fuel cell is a protonexchange membrane fuel cell.12. A method of making an Me-N—C catalyst, the method comprising:

(a) providing an N-doped carbon substrate comprising a metal M in M-N₄moieties;

(b) contacting the N-doped carbon substrate with a vapor comprising Me,whereby Me-N₄ moieties form on the N-doped carbon substrate and a vaporcomprising the metal M is released from the N-doped carbon substrate.

13. The method of feature 12, wherein the vapor comprising Me is a vaporcomprising Me(Ha)x, wherein:

X is 2, 3, or 4;

Ha is a halide anion, an organic anion, or a combination thereof; and

Me is a transition metal.

14. The method of feature 13, wherein Me(Ha)x has a boiling point ofless than about 900° C.15. The method of any one of features 12-14, wherein the vaporcomprising Me is carried with an inert gas.16. The method of any one of features 12-15, wherein the contacting isat a temperature in the range from about 600° C. to about 900° C.17. The method of feature 16, wherein the temperature is about 750° C.18. The method of any one of features 12-17, wherein the contacting isfor about 3 hours.19. The method of any one of features 12-18, wherein Me is Fe, Mn, Co,or a combination thereof.20. The method of any one of features 12-19, wherein Ha is chlorine,bromine, or a combination thereof.21. The method of any one of features 13-20, wherein Me(Ha)x is FeCl₃.22. The method of any one of features 12-21, wherein the metal M is Zn,Cd, or a combination thereof.23. The method of feature 22, wherein the metal M is Zn.24. The method of any one of features 12-23, wherein the vaporcomprising the metal M is a vapor comprising M(Ha)x, wherein; X is 2, 3,or 4; and Ha is a halide, an organic anion, or a combination thereof.25. The method of feature 24, wherein M(Ha)x has a boiling point of lessthan about 900° C.26. The method of any one of features 12-25, wherein the vaporcomprising M is a vapor comprising ZnCl₂.27. The method of any one of features 12-26, wherein a vapor comprisinga halide is released from the N-doped carbon substrate.28. The method of feature 27, wherein the vapor comprising a halidecomprises Cl₂.29. The method of any one of features 13-29, wherein step (b) comprisespyrolyzing the N-doped carbon substrate and a material comprisingMe-(Ha)x, wherein X=2-4, such that at least a portion of the Me(Ha)xvaporizes to a vapor comprising Me(Ha)x, whereby the vapor contacts theN-doped carbon substrate and Me-N₄ sites form on the N-doped carbonsubstrate.30. The method of feature 29, wherein Me(Ha)x is FeCl₃, and Fe—N₄ sitesform on the N-doped carbon substrate.31. The method of feature 30, wherein the vapor comprising FeCl₃ isprovided by vaporizing anhydrous FeCl₃ in a furnace.32. The method of feature 31, wherein the vaporizing comprises placing amaterial comprising FeCl₃ in an inert gas flow upstream of the N-dopedcarbon substrate.33. The method of any one of features 12-32, wherein M is Zn and aZn-halide vapor is released from the N-doped carbon substrate during theformation of Me-N₄ or Fe—N₄ sites.34. The method of feature 33, wherein the Zn-halide is ZnCl₂.35. The method of any one of features 12-34, further comprisingpurifying the Fe—N—C catalyst.36. The method of feature 35, wherein the purifying comprises removal ofFe with a magnet.37. The method of any one of features 12-36, wherein the N-doped carbonsubstrate is prepared by a method comprising:

mixing Zn(NO₃) and 2-methylimidazole in a methanol solution until asuspension comprising a zeolitic imidazolate framework eight forms;

isolating the zeolitic imidazolate framework eight; and

optionally pyrolyzing the zeolitic imidazolate framework eight.

38. The method of any one of features 12-37, wherein the N-doped carbonsubstrate is prepared by a method comprising:

mixing a zeolitic imidazolate framework eight with 1,10 phenanthrolinein a solution of ethanol and water to form a solid suspension; and

pyrolyzing the dried solid suspension under an inert gas.

39. The method of feature 37 or feature 38, wherein the N-doped carbonsubstrate is prepared by pyrolyzing under an inert gas at about 1050° C.for about one hour.40. The method of any one of features 12-39, wherein the N-doped carbonsubstrate has a Brunauer-Emmett-Teller area of at least about 800m²·g⁻¹.

As used herein an “inert atmosphere” refers to a gaseous atmosphere thatcontains little or no oxygen and can include one or more inert ornon-reactive gases, such as He, Rd, Ne, Ar, Xe, N, or a combinationthereof. The various methods disclosed herein can be carried out, eitherpartially or fully, under an inert atmosphere, which can be under vacuumor under pressure.

As used herein, “pyrolysis” refers to heating of one or more materialsat an elevated temperature. Pyrolysis can be performed in an inertatmosphere. Pyrolysis can result in thermal decomposition of one or morecompounds or materials. Pyrolysis can result in sublimation, boiling, achemical reaction, or a combination thereof.

As used herein, “transition metal salts” are metal salts in which themetal ions are transition metal ions, or metals in the d-block of theperiodic table of the elements, including the lanthanide and actinideseries. Transition metal salts include salts of scandium, titanium,vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc,yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium,palladium, silver, cadmium, lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, lutetium, hafnium, tantalum,tungsten, rhenium, osmium, iridium, platinum, gold, mercury, actinium,thorium, protactinium, uranium, neptunium, plutonium, americium, curium,berkelium, californium, einsteinium, fermium, mendelevium, nobelium, andlawrencium.

As used herein, the term “about” refers to a range of within plus orminus 10%, 5%, 1%, or 0.5% of the stated value.

As used herein, “consisting essentially of” allows the inclusion ofmaterials or steps that do not materially affect the basic and novelcharacteristics of the claim. Any recitation herein of the term“comprising”, particularly in a description of components of acomposition or in a description of elements of a device, can beexchanged with the alternative expression “consisting of” or “consistingessentially of”.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example of a method for synthesizing Fe—N₄ sites bycontacting an N—C zeolitic imidazolate framework with gaseous FeCl₃ andreleasing ZnCl₂. FIG. 1B shows an illustration of a traditional mixingmethod, not utilizing chemical vapor deposition, wherein Fe is mixedwith a zeolitic imidazolate framework, for synthesizing Fe—N—Ccatalysts.

FIG. 2A shows a transmission electron microscope image of a zeoliticimidazolate framework synthesized utilizing the technology herein. Thescale bar at lower right is 100 nm. FIG. 2B shows a comparison of an XRDpattern of a synthesized zeolitic imidazolate framework (grey, uppertrace) and a simulated XRD pattern for zeolitic imidazolate frameworkfrom the Joint Committee on Powder Diffraction Standards (JCPDS) cardnumber 00-062-1030 (black, lower trace). FIG. 2C shows a Zn K-edgeFT-EXAFS (Fourier-transform extended X-ray absorption fine structure)spectrum and fit of an N—C zeolitic imidazolate framework. FIG. 2D showshigh-resolution Nis XPS (X-ray photoelectron spectroscopy spectrum) of aN—C zeolitic imidazolate framework. FIG. 2E shows a SEM (scanningelectron microscope) image of an N—C zeolitic imidazolate framework. Thescale bar at lower right is 1 micron. FIG. 2F shows a SEM image of anN—C zeolitic imidazolate framework. The scale bar at lower right is 300nm. FIG. 2G shows an ADF-STEM (annular dark-field scanning transmissionelectron microscopy) image of an N—C zeolitic imidazolate framework withscale bar at lower left of 200 nm. FIG. 2H shows an ADF-STEM image of anN—C zeolitic imidazolate framework with scale bar at lower left of 100nm. FIG. 2I shows an ADF-STEM image of an N—C zeolitic imidazolateframework with scale bar at lower left of 20 nm. FIG. 2J shows anADF-STEM image of an N—C zeolitic imidazolate framework with scale barat lower left of 5 nm. FIG. 2K shows an ADF-STEM image of an N—Czeolitic imidazolate framework, residual Zn atoms are circled, withscale bar at lower left of 2 nm.

FIG. 3 shows H₂—O₂ PEMFC polarization curves with and withoutiR-correction for FeNC-CVD-750. The grey dotted line (bottom trace)represents the high frequency resistance.

FIG. 4 shows Tafel plots derived from the ORR polarization curves shownin FIG. 3 illustrating the measured ORR activity at 0.9 V versus the DOE2025 target.

FIG. 5A shows an ORR polarization curve of an FeNC-CVD-750 catalyst,acquired with a steady-state rotating disk electrode (RDE) polarizationplot, (lower curve) compared to an ORR polarization curve of an N—Czeolitic imidazolate framework (N—C, upper curve). FIG. 5B shows a Tafelplot derived from the FeNC-CVD-750 ORR polarization curve shown in FIG.5A. FIG. 5C shows a comparison of the H₂—O₂ PEMFC activity at 0.9V_(iR-free) and 0.8 V_(iR-free) of FeNC-CVD-750 compared with literaturevalues. The literature values and the corresponding reference numbersshown were directly collected from (Osmieri, L, Park, J, et al. 2020).The data points represented by squares were collected at 100% RH, 1.0bar partial pressure of H₂ and O₂, 80° C. The data points represented bycircles were collected at 100% RH, 2.0 bar partial pressure of H₂ andO₂, 80° C. FIG. 5D shows the H₂-air PEMFC of an FeNC-CVD-750 catalyst.

FIG. 6A shows a SEM image of FeNC-CVD-750. The scale bar at lower leftis 300 nm. FIG. 6B shows an ADF-STEM image of FeNC-CVD-750. The scalebar at lower left is 20 nm. FIG. 6C shows a SE-STEM (secondary electronscanning transmission electron microscopy) image of FeNC-CVD-750. Thescale bar at lower left is 20 nm. FIG. 6D shows an atomic resolutionADF-STEM image of FeNC-CVD-750. The scale bar at lower left is 5 nm.FIG. 6E shows a high-resolution Nis XPS spectrum. Assignments of Nspecies are according to Artyushkova 2020. FIG. 6F shows ahigh-resolution XPS Fe_(2p) spectrum fitted with Fe(III) (90%) andFe(II) (10%) species. FIG. 6G shows an EEL (electron energy loss)spectrum showing N K-edge and Fe L-edge acquired from single atom (theatom is the circled dot in the inset where the atomic-resolution AC-STEMimage is displayed with a scale bar of 1 nm). FIG. 6H shows a Mössbauerspectrum measured at 5 K fitted with D1 (89%) and D3 (11%). FIG. 6Ishows an ex situ XANES (X-ray absorption near edge structure) spectrumof FeNC-CVD-750 with those of Fe(II)Pc and Fe(III)Pc-O₂ standards forcomparison. FIG. 6J shows an ex situ FT-EXAFS spectrum of FeNC-CVD-750and the fit with a molecular model of O₂—Fe(III)-N₄ depicted in theinset, wherein the center large grey ball represents an iron atom, thedarker smaller balls extending at the top and the bottom representoxygen atoms, and the smaller four grey balls extending axially orhorizontally represent nitrogen atoms.

FIG. 7A shows cyclic voltammograms (CVs) of FeNC-CVD-750 and N—Czeolitic imidazolate framework collected in Ar-saturated 0.5 M H₂SO₄electrolyte at room temperature with a scan rate of 10 mV·s-1. FIG. 7Bshows in situ XANES of FeNC-CVD-750 collected in O₂-purged 0.5 M H₂SO₄at room temperature as a function of potential during the anodic-goingscan. FIG. 7C shows in situ FT-EXAFS of FeNC-CVD-750 collected inO₂-purged 0.5 M H₂SO₄ at room temperature as a function of potentialduring the anodic-going scan. FIG. 7D shows a comparison of the fractionof Fe(III) over the total amounts of Fe [ΘFe(III)] as a function ofpotential derived from the linear combination analysis (LCA) of theXANES spectra (circles) and from Equation 3 with a redox potential(Eredox) of 0.66 V.

FIG. 8A shows an ADF-STEM image of a FeNC-CVD-750 cathode of the MEA(membrane electrode assembly). The scale bar at lower left is 500 nm.FIG. 8B shows a corresponding energy dispersive X-ray spectroscopy (EDS)spectrum image of the FeNC-CVD-750 cathode of the MEA shown in FIG. 8A.The lighter regions represent the C from the ionomer. The scale bar atlower left is 500 nm. FIG. 8C shows a corresponding EDS spectrum imageof the FeNC-CVD-750 cathode of the MEA shown in FIG. 8A. The lighterregions represent the Fe from the ionomer. The scale bar at lower leftis 500 nm. FIG. 8D shows a corresponding EDS spectrum image of theFeNC-CVD-750 cathode of the MEA shown in FIG. 8A. The lighter regionsrepresent the F from the ionomer. The scale bar at lower left is 500 nm.

FIG. 9 shows an FT-EXAFS spectrum of a synthesized zeolitic imidazolateframework at the Zn K-edge with the corresponding fit. Fitting resultsare listed in Table 1.

FIG. 10 shows a steady-state RDE polarization in O₂-saturated 0.5 MH₂SO₄ at room temperature using a rotation rate of 900 rpm, 20 mVpotential steps from 0.05 to 0.95 V, and a 25 s potential hold time ateach step for FeNC-CVD-650, FeNC-CVD-750, FeNC-CVD-900, andFeNC-CVD-1000.

FIG. 11A shows the first three forward scans of the H₂—O₂ PEMFCpolarization with iR-correction for FeNC-CVD-750. FIG. 11B shows thefirst three forward scans of the H₂—O₂ PEMFC polarization withiR-correction enlarged in the 0.86 to 0.94 volt region of the Y-axis ofFIG. 11A, for FeNC-CVD-750.

FIG. 12A shows XRD spectra of N—C zeolitic imidazolate framework (uppergrey trace) and FeNC-CVD-750 (lower black trace). FIG. 12B shows XPS Nisspectra of N—C zeolitic imidazolate framework (lighter grey trace) andFeNC-CVD-750 (darker black trace).

FIG. 13A shows a high resolution ADF-STEM image of FeNC-CVD-750; asquare with a crosshair highlights single atoms selected for the EELspectrum in FIG. 13B. The scale bar at lower left is 2 nm. FIG. 13Bshows an EEL spectrum of the N k-edge and Fe L-edge acquired from singleatoms highlighted in FIG. 13A.

FIG. 14A shows an ADF-STEM image of the cathode of an MEA. The scale barat lower left is 0.5 micron. FIG. 14B shows an ADF-STEM image of thecathode of an MEA. The scale bar at lower left is 200 nm.

FIG. 15A shows micropore and mesopore size distributions forFeNC-CVD-750 (darker black plot) and N—C zeolitic imidazolate framework(lighter grey plot). The dV/d log (D) is the differential pore volumedistribution, where V is pore volume and D is pore diameter. FIG. 15Bshows cyclic voltammograms of the FeNC-CVD-750 (top and bottom traceswith the peaks about 0.65V) and N—C zeolitic imidazolate framework(interior traces with no peaks) collected in an Ar-saturated 0.5 M H₂SO₄electrolyte at room temperature with a scan rate of 10 mV·s-1.

DETAILED DESCRIPTION

The present technology provides methods directed to chemical vapordeposition (CVD) synthesis of Me-N—C catalysts, wherein Me can include atransition metal, Mn, Fe, Co, or a combination of metals. The methodscan utilize non-solid-contact pyrolysis wherein a metal salt can bevaporized. Gaseous metal from the vaporized metal salt can be depositedinto N-doped carbon (N—C zeolitic imidazolate framework) defects.Gaseous metal from the vaporized metal salt can displace a metal M fromthe N—C zeolitic imidazolate framework. The non-solid-contact pyrolysisdoes not mix solid iron precursors (e.g., Me=Fe) with the solid N—Czeolitic imidazolate framework precursors during or before thesynthesis, which improves the process compared to conventional methods.

The non-solid-contact pyrolysis can be conducted at about 750° C., lowerthan the conventional process at about 1000° C., and the lowertemperature can produce more Me-N₄ sites. The Me-N₄ sites can readilydecompose at temperatures of 1000° C. or more. The Me-N—C catalystsformed can have active Fe—N₄ sites that are formed via vapor depositionof gas phase FeCl₃ into defects in the N—C zeolitic imidazolateframework material. The FeCl₃ can displace a metal M from the N—Czeolitic imidazolate framework material. The use of anhydrous FeCl₃ asthe iron precursor, due to its low boiling point (about 316° C.), canpromote the success of the CVD method disclosed herein.

Another key the success of the CVD method is that the N—C zeoliticimidazolate framework can be optimized by preparation at a highertemperature (e.g., about ≥1000-1050° C.) before the CVD process. Duringthe CVD synthesis, the gaseous metal can be directed towards the N—Czeolitic imidazolate framework, for example, by placing the source ofthe gaseous metal upstream in an inert gas flow from the N—C zeoliticimidazolate framework.

The technology herein can overcome the limits of replacing scarce andexpensive platinum (Pt) with metal-nitrogen-carbon (Me-N—C) catalystsfor the oxygen reduction reaction in proton exchange membrane fuelcells, which has largely been impeded by the low activity of Me-N—C, inturn limited by low site density and low site utilization. For example,the technology herein can implement chemical vapor deposition tosynthesize an Fe—N—C catalyst, in an approach fundamentally differentfrom previous synthetic routes. The technology disclosed herein canclose the activity gap between Pt and Fe—N—C catalysts, and thus allowsfor replacing Pt with inexpensive and earth-abundant materials (e.g.,iron) in the PEMFC stack in vehicles. The present catalysts can be usedas the cathode catalyst in the PEMFC in hydrogen fuel cells for vehiclesand for stationary applications. The catalysts also can be used for CO₂reduction and in direct methanol fuel cells, since they have high ORRactivity and immunity to CO poisoning. The catalysts also can be used asthe cathode catalyst in alkaline fuel cells.

Me-N—C catalysts can be prepared by providing an N-doped carbonsubstrate including a metal M in M-N₄ moieties. The metal M can be anymetal incorporated in a metal organic framework. The N-doped carbonsubstrate can be contacted with a vapor including a Me-halide, whereinMe is a transition metal (e.g., Mn, Fe, Co, or a combination of metals).The halide can include anions, for example anions including F, Cl, Br,I, At, Ts, or a combination thereof.

When the N-doped carbon substrate is contacted with a vapor includingthe Me-halide, the metal M in the N-doped carbon substrate (in M-N₄moieties) can be displaced by the metal Me in the Me-halide. Me-N₄moieties can form on an exterior surface of the N-doped carbonsubstrate. The displacement can include formation of a second metal (M)halide vapor, or M-halide gas. A second M-halide can vaporize from theN-doped carbon substrate. A CVD method can be represented by Reaction 1.

Me-Halide (gas)+M-N₄ (solid)→Me-N₄ (solid)+M-Halide (g)   Reaction 1

The temperature of the CVD method in Reaction 1 can be selected so thatthe Me-Halide and the M-Halide form a vapor at the selected temperature.The temperature can be sufficiently low to ensure that the N-dopedcarbon substrate does not decompose and the Me-N₄ (solid) sites do notdecompose. For example, if Me=Fe, M=Zinc, and Halide=chlorine, theboiling point of FeCl₃ is about 316° C., the boiling point of ZiCl₂ isabout 732° C., and the boiling point of Cl₂ is about −34° C. At atemperature of about 750° C., a vapor including FeCl₃ can contact Zn—N₄(solid, in the N—C zeolitic imidazolate framework), react with Zn ordisplace Zn, and release ZnCl₂ gas. Cl₂ can also be released as depictedby Halide₂ (gas) in Reaction 2 below.

A variety of oxidation states of the metal M and the transition metal Mecan be represented in the example depicted in Reaction 2 below. Forexample, if the transition metal Me has a +3 oxidation state at theoutset, a CVD method can be represented by Reaction 2.

2Me-Halide₃ (gas)+2M-N₄ (solid)→2Me-N₄ (solid)+2M-Halide₂ (gas)+Halide₂(gas)   Reaction 2

The technology can include anions in addition to halide anions. Thehalide can include non-halide anions, for example, SO₄, CO₃, anysuitable anion, or a combination thereof. The CVD reaction can include asuitable metal Me with a suitable anion (halide or other anion) vapor,which contacts an N-doped carbon substrate to form a Me-N—C catalyst.The metal M in the N-doped carbon substrate can be displaced by themetal Me. The temperature can be sufficiently high to provide a vaporincluding the Me-halide (or other anion). For example, FeSO₄ can form agas at about 330° C. A vapor including FeSO₄ can contact an N—C zeoliticimidazolate framework including Zn. ZnSO₄ can form a vapor at about 740°C. The iron in the vapor including FeSO₄ can displace the Zn, to formMe-N₄ moieties on the N-doped carbon substrate. Various temperatures canbe utilized in the CVD methods. For example, anhydrous MnSO₄ boils atabout 850° C. A vapor including MnSO₄ can contact the N—C zeoliticimidazolate framework. ZnSO₄ can form a vapor at about 740° C. Themanganese in the vapor including MnSO₄ can displace the Zn, to formMe-N₄ moieties on the N-doped carbon substrate.

An example of a CVD method is depicted in FIG. 1A. FIG. 1A depicts avapor including FeCl₃ contacting an N—C zeolitic imidazolate framework(left) at about 750° C. Gaseous ZnCl₂ is released at the right of FIG.1A. Although not shown in FIG. 1A, Cl₂ gas can be released as Fedisplaces Zi from Zi—N₄ sites in the N—C zeolitic imidazolate framework.Fe—N₄ moieties can form on the exterior of the N—C zeolitic imidazolateframework. The Fe—N₄ moieties can be formed at about 750° C., allowingincreased nitrogen content and increased Fe—N₄ sites. Iron clustering,for example, in Fe particles, can also be minimized. The Fe—N₄ sites canbe located exclusively or about exclusively on the outer-surface of theN—C zeolitic imidazolate framework material. After the method isconducted, the method can include a step for removing residual Fe, forexample, by passing a magnet over the synthesized catalyst.

An example of a traditional mixing method, not utilizing chemical vapordeposition, wherein Fe is mixed with a zeolitic imidazolate framework,for synthesizing Fe—N—C catalysts, is depicted in FIG. 1B. The zeoliticimidazolate framework (ZIF) is mixed with iron at the left of FIG. 1B.The iron is distributed throughout the ZIF at the center of FIG. 1B.Pyrolizing at 1000° C.±100° C. can form some Fe—N₄ sites throughout theZIF (right), with a small proportion of Fe—N₄ sites accessible at theexterior surface of the ZIF. The small proportion of Fe—N₄ sitesaccessible at the exterior surface of the ZIF do not provide Fe—N₄ siteslocated exclusively or about exclusively on the outer-surface of the N—Czeolitic imidazolate framework material.

The N-doped carbon (N—C zeolitic imidazolate framework) utilized for themethods described herein can be made without ball milling the N—Czeolitic imidazolate framework after it has been pyrolized, which cancontribute to the performance of the final Me-N—C catalyst. To preparethe N—C zeolitic imidazolate framework, a zeolitic imidazolate frameworkeight (ZIF-8) can be made by adding an about 0.1 M Zn(NO₃).6H₂O methanolsolution to an about 0.4 M 2-methylimidazole methanol solution, withstirring for about one hour at room temperature. The resultingsuspension can be collected and washed by centrifugation using methanolthree times, and later dried at 40° C. in a vacuum oven overnight, toform a ZIF-8. FIG. 2A shows a transmission electron microscope image ofa ZIF-8, with a scale bar at lower right of 100 nm. An X-ray diffractionpattern of the synthesized ZIF-8 (FIG. 2B, top) matches that of thepattern in the XRD database (JCPDS: 00-062-1030, FIG. 2B bottom),verifying the formation of ZIF-8.

The ZIF-8 and 1,10 phenanthroline can be dispersed in a solution ofethanol and deionized water with a volume ratio of about 2:1. Thesuspension can be dried in a vacuum oven overnight. The resulting drypowders can be ball milled for about 3 hours in a plastic container withplastic balls. The resulting powders can be pyrolyzed under an inert gasat 1050° C. for about one hour after reaching 1050° C. with a ramp rateof 5° C. per minute, followed by unassisted cooling to room temperature.The powders collected can be referred to as an N—C zeolitic imidazolateframework and can be used for the subsequent non-solid-contact pyrolysis(CVD method). FIG. 2C depicts fitting of Zn K-edge EXAFS data for a N—Czeolitic imidazolate framework. Table 1 presents the Zn—N bond distance,shown as R(A). As shown in FIG. 2C and in Table 1, fitting of the ZnK-edge EXAFS data for the N—C zeolitic imidazolate framework shows thatZn is present in the form of Zn—N₄ with a Zn—N bond distance of2.00±0.01 Å.

The existence of Zn—N binding in the N—C zeolitic imidazolate frameworkis also shown by fitting of the X-ray photoelectron spectroscopy (XPS)Nis spectrum shown in FIG. 2D, wherein the shaded peak at ˜399.5 eV iscommonly assigned to N bonded to a transition metal (Artyushkova, 2020).

The scanning electron microscopy images of a N—C zeolitic imidazolateframework shown in FIG. 2E (scale bar at lower right=1 micron) and inFIG. 2F (scale bar at lower right=300 nm) demonstrate that the size andshape of the crystals of ZIF-8 (FIG. 2A) can be preserved afterpyrolysis and transformation into N—C zeolitic imidazolate frameworkparticles, despite a possible loss of the mass during formation of theN—C zeolitic imidazolate framework.

Single Zn atoms embedded in the highly porous N—C zeolitic imidazolateframework can be directly visualized using aberration-corrected scanningtransmission electron microscopy in annular dark-field images (FIGS.2G-2K). In FIG. 2K, which has a scale bar of 2 nm, residual Zn atoms arecircled. An N—C zeolitic imidazolate framework can include abundantZn—N₄ moieties embedded in a highly porous carbon matrix.

A Me-halide (e.g., FeCl₃) can be provided in a gas or vapor thatcontacts the N—C zeolitic imidazolate framework at a temperature. Thetemperature can be varied, for example, if vacuum or pressure conditionsare utilized for CVD. If different transition metal salts are utilized,the conditions and/or temperatures can be optimized. If a metal M otherthan Zn is utilized in the N—C zeolitic imidazolate framework, themethod can be optimized to provide for displacement of the metal M bythe metal Me.

To perform non-solid-contact pyrolysis during a CVD method, for example,anhydrous FeCl₃ can be placed in a boat in a pyrolysis tube at theupstream of an inert gas flow. The N—C zeolitic imidazolate frameworkcan be placed in another boat in the form of a thin layer in the boat,and the boat is also placed into the pyrolysis tube, with a gap betweenthe two boats. The pyrolysis furnace is heated up to about 750° C. witha ramp rate of 25° C. per minute, and then the temperature is held atabout 750° C. for about three hours, followed by cooling down to roomtemperature naturally. The resulting powders are then collected from thefurnace and subjected to purification by, for example, moving a magnet˜0.5 cm above the powders to remove Fe particles. The purified powderscan then be utilized as catalysts. FIG. 3 shows the purified powderstested as a catalyst.

The H₂—O₂ PEMFC polarization curves with and without IR-correction areshown in FIG. 3 and are measured with the cathode having approximately6.0 mg-cm⁻² of the catalyst and an anode having about 0.3 mg_(Pt)·cm⁻²Pt/C. The membrane is Nafion 212, and 200 mL/min⁻¹ gas is fed at bothanode (H₂) and cathode (O₂) with 100% relative humidity and 1.0 barpartial pressure each side. The cell is at 80° C., and the electrodearea is 5 cm². In FIG. 4 , the Tafel plot derived from the IR-correctedORR polarization curve displayed in FIG. 3 is shown to manifest theactivity at 0.9 V in comparison with the DOE 2025 target (the DOE 2025target is 44 mA·cm⁻² at 0.90 V).

When different temperatures, for example 650° C., 750° C., 900° C., and1000° C. are utilized during contacting an N—C zeolitic imidazolateframework with FeCl₃, the catalyst FeNC-CVD-750 (synthesized at 750° C.)exhibits the highest ORR activity (FIG. 10 ) with a half wave potentialof 0.85 V (all potentials are versus reversible hydrogen electrode). Toacquire the data shown in FIG. 10 , steady-state RDE polarization inO₂-saturated 0.5 M H₂SO₄ at room temperature using a rotation rate of900 rpm, 20 mV potential steps from 0.05 to 0.95 V, and a 25 s potentialhold time were used at each step. FIG. 5A shows the ORR polarizationcurve of the FeNC-CVD-750 catalyst (lower curve) compared to the ORRpolarization curve of the N—C zeolitic imidazolate framework (N—C, uppercurve). The data presented in FIG. 5A was acquired using a catalystloading of 800 μg·cm⁻² and oxygen-saturated 0.5 M H₂SO₄.

The corresponding kinetic current density for an FeNC-CVD-750 catalystderived from the current at 0.8 V and the limiting current using theKoutecky-Levich equation is 20 mA·cm⁻² or 25 A·g⁻¹ as depicted in FIG.5B. The half-wave potential and mass activity values are among thehighest reported for PGM-free catalysts in acidic electrolyte (Beltren,D E, and Litster, S, 2019). The kinetic current density exhibits a Tafelslope of ˜60 mV/dec above 0.8 V (FIG. 5B), which has been commonlyreported for Fe—N—C catalysts (Osmieri, L, et al. 2019; Li, J, et al.,2017). This Tafel slope is comparable to that of Pt-based catalysts(Gasteiger, H A, et al., 2005), indicating that they share the same ratedetermining step for the ORR.

An FeNC-CVD-750-containing electrode evaluated in an H₂—O₂ PEMFC forthree full polarization scans, followed by acquisition of H₂-airpolarization curves on the same membrane electrode assembly (MEA) showsunexpected results. A current density of 44 mA·cm⁻² is reached at 0.89V_(iR-free) during the first scan (increasing-current) in the H₂—O₂PEMFC at 1.0 bar partial pressure of O₂ and 80° C., 0.01 V lower thanthe DOE 2025 target (FIG. 3 and FIG. 4 ). In FIG. 3 , the cathodeincludes: ˜6.0 mg·cm⁻² of an FeNC-CVD-750 catalyst; the anode: 0.3mg_(Pt)·cm⁻² Pt/C; the membrane: Nafion 212; 200 mL·min⁻¹ gas fed at theanode (H₂) and 1000 mL·min⁻¹ at the cathode (O₂) at 100% RH, 1.0 barpartial pressure H₂ and O₂, 80° C., electrode area 5 cm². A currentdensity of 33 mA·cm⁻² is achieved at 0.9 V_(iR-free) and 380 mA·cm⁻² at0.8 V_(iR-free), both exceeding those of all previous PGM-free catalystsreported to date in H₂—O₂ PEMFCs under similar conditions (FIG. 5C).

When repeated scans are utilized, the current at 0.9 V_(iR-free) dropsto 22 mA·cm⁻² and then 18 mA·cm⁻² on the second and third scans,respectively (FIG. 11A and FIG. 11B) indicating that the FeNC-CVD-750catalyst has poor stability in H₂—O₂ PEMFCs, similar to highly activeFe—N—C catalysts (Shao, Y, et al., 2019; Osmieri, L, et al., 2020). Theconditions for FIGS. 11A and 11B are cathode: ˜6.0 mg·cm⁻² of theFeNC-CVD-750 catalyst; anode: 0.3 mg_(Pt)cm⁻² Pt/C. This result isexpected since Fe—N—C catalysts, including FeNC-CVD-750, can share thesame Fe—N₄ active sites. Despite the degradation, a maximum powerdensity of 0.37 W·cm⁻² was obtained in the subsequent H₂-air PEMFCtesting (FIG. 5D), which is among the highest values reported forPGM-free catalysts thus far.

A representative SEM image of FeNC-CVD-750 (FIG. 6A) shows a similarpowder morphology to the N—C zeolitic imidazolate framework (FIG. 2F),without noticeable particle growth and aggregation. The ADF- andsecondary electron (SE)-STEM images, at different magnifications, showthe highly porous morphology of the carbon matrix and absence of metalclusters (FIG. 6B-6D). The XPS Nis spectrum of the FeNC-CVD-750 (FIG.6E) is nearly the same as that of the N—C zeolitic imidazolate framework(FIG. 2D). The XRD pattern of the FeNC-CVD-750 (FIG. 12A) and the N—Czeolitic imidazolate framework (“N—C”, FIG. 12A), and the C and Ncontents (Table 2), are nearly the same, comparing the FeNC-CVD-750results to those of the N—C zeolitic imidazolate framework. Theseresults show that the overall morphology of the N—C zeolitic imidazolateframework can be largely preserved after the CVD at about 750° C. Thiscan be expected considering that the N—C zeolitic imidazolate frameworkwas synthesized using pyrolysis at about 1050° C. prior to the CVD ofFe. FIG. 6F depicts a high-resolution XPS Fe_(2p) spectrum fitted withFe(III) (90%) and Fe(II) (10%) species compared to empirically acquiredXPS for the FeNC-CVD-750.

In the FeNC-CVD-750, the Zn content drops from 2.16 wt % in the N—Czeolitic imidazolate framework to 0.12 wt % in FeNC-CVD-750, accompaniedby incorporation of 2.00 wt % Fe (see Table 2). The presence of abundantFe—N_(x) moieties in FeNC-CVD-750 is directly evidenced by atomicresolution ADF-STEM imaging coupled with electron energy lossspectroscopy (EELS). Abundant bright dots are clearly seen in theADF-STEM image (FIG. 6G, inset), for which the EELS point spectrum showsthe close proximity of single Fe atom and N (FIG. 6G, also FIG. 13A andFIG. 13B).

The presence of Fe—N₄ moieties in FeNC-CVD-750 is also supported by the⁵⁷Fe Mössbauer spectrum collected at 5 Kelvin (FIG. 6H). The lowestpossible temperature during Mössbauer data acquisition is important todistinguish superparamagnetic Fe species (such as nano-Fe-oxides) fromFe—N₄ sites. While both nano-Fe-oxides and O₂—Fe(III)-N₄ sites lead to asimilar D1 signal at room temperature, this degeneracy is usuallyunveiled at 5 K: nanosized superparamagnetic Fe oxides convert into asextet component while O₂—Fe(III)-N₄ sites still contribute with a D1component (Li, J, Jia, Q, et al., 2019).

The ⁵⁷Fe Mössbauer spectrum at 5K of FeNC-CVD-750 identifies twodoublets, labelled D1 and D3, representing 89% and 11% of the absorptionarea, respectively (FIG. 6H and Table 3). For example, FIG. 6I shows anex situ XANES spectrum of FeNC-CVD-750 with those of Fe(II)Pc andFe(III)Pc-O₂ standards for comparison. FIG. 6J shows an ex situ FT-EXAFSspectrum of FeNC-CVD-750 and the fit with a molecular model ofO₂—Fe(III)-N₄ depicted in the inset, wherein the center large grey ballrepresents an iron atom, the darker smaller balls extending at the topand the bottom represent oxygen atoms, and the smaller four grey ballsextending axially or horizontally represent nitrogen atoms.

The synthesized FeNC-CVD-750 catalysts can show unprecedented ORRactivity in a H₂—O₂ PEMFC. The catalysts have ultra-dense Fe—N₄ sites asreflected by the high intensity of the Fe³⁺/²⁺ redox peaks (FIG. 7A).The Fe wt % of the catalysts was estimated by inductively coupled plasmaand is about 2 wt % (Table 2), which can be more than catalysts producedby conventional methods, particularly when the Fe at an exterior surfaceof the catalyst is considered. The surface deposition feature of themethods disclosed herein forms enriched Fe—N₄ sites on the surface andthus improves the PEMFC performance.

To confirm that all the Fe—N₄ sites in FeNC-CVD-750 areelectrochemically active during the ORR, in situ XAS was conducted onFeNC-CVD-750 at the Fe K-edge in an O₂-purged 0.5 M H₂SO₄ electrolyte ina flow cell as a function of applied potential. The XANES and FT-EXAFSspectra collected at 0.9 V nearly overlap that of Fe(III)Pc-O₂ (FIG.7B), which confirms that the vast majority of Fe—N₄ sites are in theform of Fe(III)-N₄—O₂ at 0.9 V (see Table 4). As the potential isgradually reduced to 0.5 V, the XANES spectrum shifts negatively, andcorrespondingly the intensity of the FT-EXAFS peak drops (FIG. 7C).These occurrences have been commonly observed on Fe—N—C catalysts andascribed to the redox transition from Fe(III)-N₄—O₂ to Fe(II)-N₄(Zitolo, A, et al., 2017, Li, J, et al., 2016, Osmieri, L, et al., 2019,Li, J, et al., 2017). However, the FT-EXAFS spectrum at 0.5 V exhibits ashoulder around 1.8 Å, rather than just the one prominent peak observedat 0.9 V (FIG. 7C), and it cannot be fitted with an Fe—N₄ model. Asdepicted in FIG. 7D, the ΘFe(III)(E) acquired by the linear combinationanalysis (LCA) closely follows the one calculated from Equation 3, whichis described below.

Some leading PGM-free ORR catalysts are transition metal-nitrogen-carbon(Me-N—C, e.g., M=Fe or Co) materials (Li, J, et al., 2018; Zitolo, A, etal., 2017; Zhang, H, et al., 2017). Highly active Fe—N—C catalysts havebeen produced by various methods such as hard templating (silica)(Serov, A, et al., 2015; Wan, X, et al., 2019) and soft templating(polymer and organic compounds) (Chung, H T, et al. 2017; Tylus, U, etal., 2014), of Zn-based metal organic framework (Zhang, H, et al.,2019). All these methods incorporate the core feature of the synthesisroute initiated by Gupta, S and Yeager, et al., in 1989, that is,pyrolyzing at 900-1100° C. a catalyst precursor that includes mixed Fe,N, and C elements.

From structural characterizations, it has been identified that all thepyrolyzed Fe—N—C catalysts share similar Fe—N₄ sites (Li, J, et al.,2016), formed during the pyrolysis step (Li, J, et al., 2020). The ORRactivity in acid medium of these Fe—N—C catalysts is limited by both thelow turnover frequency (TOF) and low density of gas-phase accessibleFe—N₄ sites per mass of Fe—N—C(SD_(mass)) (Primbs, M, et al., 2020).Primbs et al. (2020) reported a comprehensive analysis of the catalyticoxygen reduction reaction (ORR) reactivity of four benchmark platinumgroup metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts.Primbs et al. determined the SD_(mass) via CO chemisorption and theensuing average TOF. Among this set of benchmark catalysts, both thehighest SD_(mass) (˜6×10¹⁹ sites·g⁻¹) and highest TOF (˜0.7e⁻·site⁻¹·s⁻¹ at 0.8 V) are approximately one order of magnitude lowerthan that of Pt/C (Paulus, U A, et al., 2002, Gasteiger, H A, et al.,2005). Thus, improving the TOF and/or SD_(mass) of Fe—N—C catalysts canbe effective pathways to advancing their ORR activity. It is unclear howto improve the TOF of Fe—N₄ sites prepared via pyrolysis. Developingother PGM-free sites with higher TOFs may be an alternative option.Recently, a Sn—N—C catalyst with Sn—N_(x) sites showed a similar TOFthan Fe—N₄ sites in a parent Fe—N—C catalyst prepared similarly, but alower SD_(mass) (Luo, F, et al., 2020).

Increasing the SD_(mass) of Me-N—C catalysts can be the most feasibleapproach to increase their ORR activity. It faces however at least twochallenges: i) the parallel formation during pyrolysis of Fe—N₄ sitesand ORR-inactive or less active Fe species at high Fe content (Proietti,E, et al., 2011, Zhang, H, et al., 2019), and ii) the uncontrolledlocation of Fe—N₄ sites, a fraction of them being buried in the bulk ofthe N-doped carbon matrix with current synthetic approaches, andtherefore inaccessible by O₂.

Related to the challenges i) and ii), herein are defined two utilizationsub-factors, U_(Fe), and U_(FeN4), the former being the ratio of thenumber of Fe atoms present as Fe—N4 moieties to the total number of Featoms in a Fe—NC catalyst, and the latter the ratio of gas-phaseaccessible Fe—N₄ moieties to all Fe—N₄ moieties in a catalyst. TheSD_(mass) can be related to U_(Fe), U_(FeN4), and the Fe wt % byEquation 1.

$\begin{matrix}{{SD}_{mass} = {\frac{{{Fe}{wt}}\%}{100 \times M_{Fe}} \times N_{A} \times U_{Fe} \times U_{{FeN}4}}} & {{Equation}1}\end{matrix}$

Where Fe wt % is the total Fe content in Fe—N—C, M_(Fe) is the molarmass of iron. N_(A) IS Avogadro's constant. The overall utilizationfactor, U, can be defined by Equation 2.

U=U _(Fe) ×U _(FeN4)   Equation 2

Developing a synthetic approach that favors the conversion of Fe intoFe—N₄ sites even at relatively high Fe content, while simultaneouslyfavoring the location of Fe—N₄ sites on the outer-surface, is a longsought technology.

As an example of the challenge i), Wan, X and Shui et al. (Wan, X, etal., 2019) recently showed that the U of their Fe—N—C catalystsdramatically drops from 43.5% to ˜11.4% as the Fe content increases from0.3 wt % to 2.8 wt %, due to strong Fe clustering at high Fe content.This led to a maximum SD_(mass) of 3.4×10¹⁹ sites·g¹, comparable tothose of the benchmark Fe—N—C catalysts discussed in Primbs et al.,2020.

As an example of the challenge ii), Primbs et al. (2020) showed that theSD_(mass) values measured by CO-chemisorption of Fe—N—C catalysts withFe being present only or mostly as Fe—N₄ sites are significantly lowerthan the total number of Fe—N₄ sites determined by ⁵⁷Fe Mössbauerspectroscopy. The SD_(mass) values reached only 20-45% of the bulk SD ofFe—N₄ sites (i.e. U_(FeN4)=20-45%), except for the PAJ or PajaritoPowder Inc. (pajaritopowder.com) catalyst (U_(FeN4) 80%). The latter,however, was characterized at the same time with a low U_(Fe) value,with Fe being present mainly as Fe particles (Primbs, et al., 2020). Thelow U_(Fe) and/or U_(FeN4) in Fe—N—C catalysts are related to the majorapproach for the synthesis of Fe—N—C catalysts that involves mixing orcombining Fe, N, and C precursors first, and subjecting the catalystprecursor to high-temperature pyrolysis leading to the simultaneoustransformation of N and C into a N-doped carbon matrix and of Fe, N andC into Fe—N₄ sites and/or Fe clusters. This results in the formed Fe—N₄sites being located rather uniformly mixed throughout the N-dopedmicroporous carbon (N—C) matrix. Those Fe—N₄ sites buried in the coreare electrochemically inactive, leading to U_(FeN4)<<100%.

High Fe contents can also reduce the U_(Fe) by graphitizing the N—Czeolitic imidazolate framework during pyrolysis, lowering the N-content,in turn decreasing the ability of N—C zeolitic imidazolate framework toaccommodate Fe—N₄ sites (Proietti, E, et al., 2011). The SD_(mass) ofFe—N—C catalysts is inherently limited when using the existing synthesisapproaches including mixing of materials before pyrolysis. Increasingthe SD_(mass) of Fe—N—C catalysts by developing new synthesis routes is,however, hindered by the poor understanding of the Fe—N4 site formationmechanism during pyrolysis.

During investigations herein, it has been determined that Fe—N₄ sitescan be formed through gas phase diffusion of single iron atoms (Fe₁) intetrahedral Fe-04 moieties into N₄—C cavities during pyrolysis (Li, J,et al., 2020). The ultrashort diffusion length of Fe, requires closeproximity of Fe sources and N₄—C cavities, otherwise Fe, atoms nucleateforming Fe clusters during diffusion, as observed during the pyrolysisof Fe(II) acetate that is not in physical contact with the N—C zeoliticimidazolate framework substrate (Li, J, et al., 2020). This researchexplains the necessity to sufficiently mix Fe precursor with N and Cprecursors to form Fe—N₄ sites with previous synthesis routes. Thetechnology herein avoids the mixing stage and minimizes the formation ofFe clusters by choosing Fe precursors with long diffusion lengths.

The technology herein can implement chemical vapor deposition (CVD) toflow iron chloride vapor above a bed of N—C zeolitic imidazolateframework powders to preferentially form Fe—N₄ sites on theouter-surface. The iron chloride vapor can have a long diffusion lengthbecause the Fe atoms are individually surrounded by chloride ions, whichprohibits iron atoms from nucleating during diffusion. Structural andelectrochemical characterization confirm that a high density of Fe—N₄sites are exclusively formed on the outer-surface of N—C zeoliticimidazolate framework, accessible by air, leading to full utilization ofFe—N₄ sites (U_(FeN4)=100%). A catalyst synthesized herein at 750° C.exhibits an unprecedented ORR activity of 33 mA·cm⁻² at 0.90 V_(iR-free)and 44 mA·cm⁻² at 0.89 V_(iR-free) in an H₂—O₂ PEMFC at 1.0 bar and 80°C., only 0.01 V lower than the DOE 2025 target (FIG. 4 ).

An Fe—N—C catalyst, which can be prepared by flowing iron chloride vaporabove a N—C zeolitic imidazolate framework substrate at about 750° C.,has a record Fe—N₄ site density of 2×10²⁰ sites·gram⁻¹ with 100% siteutilization. A combination of characterizations shows that the Fe—N₄sites formed via CVD are located exclusively on the outer-surface, areaccessible by air, and are electrochemically active.

EXAMPLES

Zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O, 99.0%), 2-methylimidazole(99%), methanol solution, zinc phthalocyanine (Zn(II)Pc, 97%),1,10-phenanthroline monohydrate, ethanol solution, anhydrous Iron(III)chloride (FeCl₃, 99%), iron(II) phthalocyanine (Fe(II)Pc, 95%),Iron(III) phthalocyanine-tetrasulfonic acid (Fe(III)Pc-O₂, 80%), andsulfuric acid (H₂SO₄, 95-97%, PPT Grade) were purchased fromSigma-Aldrich. All aqueous solutions were prepared using deionized (DI)water (18.2 MO/cm) obtained from an ultra-pure purification system.

Example 1: Synthesis of Zeolitic Imidazolate Framework

To obtain a highly porous and nitrogen rich substrate, a zeoliticimidazolate framework (ZIF-8) was prepared. 200 mL homogeneous 0.1 MZn(NO₃)₂.6H₂O methanol solution was added to 200 mL 0.4 M2-methylimidazole methanol solution under magnetic stirring for one hourat room temperature. The suspension was collected and washed bycentrifugation using methanol three times. The washed suspension wasthen dried at 40° C. in a vacuum oven overnight. The crystal size of theZIF-8 was a uniform ˜80 nm. FIG. 2A shows a transmission electronmicroscope image of the ZIF-8, with a scale bar at lower right of 100nm.

The X-ray diffraction (XRD) pattern of the synthesized ZIF-8 matchesthat of the pattern in the XRD database (JCPDS: 00-062-1030), verifyingthe exclusive formation of ZIF-8. FIG. 2B shows the XRD pattern of thesynthesized ZIF-8 in the top trace. In FIG. 2B, the bottom trace islabeled as the simulated ZIF-8 XRD pattern from JCPDS: 00-062-1030. Thetetrahedral Zn—N₄ structure in the ZIF-8 was confirmed by analysis ofthe Fourier-transform of the extended X-ray absorption fine structure(FT-EXAFS) spectrum at the Zn K-edge, which is shown in FIG. 9 . Table 1presents a summary of the fitting results illustrated in FIG. 9 .

Next, the nanosized ZIF-8 (1.0 g) and 0.25 g 1,10 phenanthroline weredispersed in a solution of ethanol and deionized water with a volumeratio of 2:1. The mixture was magnetically stirred for two hours andthen dried at 80° C. in a vacuum oven overnight. The dry powder was ballmilled for 3 hours in a plastic container with 5 plastic balls with adiameter of 0.25 inch (0.635 cm). The collected powders were thenpyrolyzed under Ar at 1050° C. for one hour, after reaching the 1050°C., with a ramping rate of 5° C. per minute, followed by cooling downnaturally to room temperature. The powders obtained were labelled as“N—C” zeolitic imidazolate framework.

FIG. 2C shows fitting of the Zn K-edge EXAFS data for the N—C zeoliticimidazolate framework. Table 1 presents the Zn—N bond distance shown asR(A). As shown in FIG. 2C and in Table 1, fitting of the Zn K-edge EXAFSdata for the N—C zeolitic imidazolate framework shows that Zn is presentin the form of Zn—N₄ with a Zn—N bond distance of 2.00±0.01 Å. A similarZn—N₄ structure was recently reported by Jaouen et al (Li, J; Pršlja, P,et al., 2019) in a nitrogen-doped carbon prepared by flash pyrolysis ofZIF-8 at 1050° C., with or without mixing with a second transitionmetal.

The existence of the Zn—N binding is also supported by fitting of theX-ray photoelectron spectroscopy (XPS) Nis spectrum shown in FIG. 2D,wherein the shaded peak at ˜399.5 eV is commonly assigned to N bonded toa transition metal (Artyushkova, 2020).

The N—C zeolitic imidazolate framework was shown to have aBrunauer-Emmett-Teller area of 807 m²·g⁻¹ and a microporous surface areaof 692 m²·g⁻¹. The high porosity of the N—C zeolitic imidazolateframework is a result of the high initial microporosity of the ZIF-8,plus the release of a large amount of Zn from ZIF-8 as Zn vapor duringthe pyrolysis creates abundant voids inside the ZIF-8 crystals. This issupported by the scanning electron microscopy (SEM) images that manifestthe preservation of the size and shape of the crystals of ZIF-8 (FIG.2A) after pyrolysis and transformation into N—C zeolitic imidazolateframework particles, despite the significant loss of the mass (FIG. 2Eand FIG. 2F). FIG. 2E shows an SEM image of the N—C zeolitic imidazolateframework with a scale bar of 1 micron at lower right. FIG. 2F shows andSEM image of the N—C zeolitic imidazolate framework with a scale bar of300 nm at lower right. shown. Single Zn atoms embedded in the highlyporous N—C zeolitic imidazolate framework can be directly visualizedusing aberration-corrected scanning transmission electron microscopy(AC-STEM) in annular dark-field (ADF) images (FIGS. 2G-2K). In FIG. 2K,which has a scale bar of 2 nm, residual Zn atoms are circled.Collectively, the N—C zeolitic imidazolate framework possesses abundantZn—N₄ moieties embedded in a highly porous carbon matrix.

TABLE 1 Summary of the fitting results of the FT-EXAFS spectra collectedat the Zn Kedge of the prepared ZIF-8, commercial Zn(ll)Pc, and the Znin the ZIF-8-derived N-C. Zn-N bond R(A) N σ² ×10⁻³(Å²) E₀ (eV) ZIF-81.99(1) 4.1(5) 6(2) 4(1) Zn(ll)Pc 1.98(1) 3.8(8) 4(2) 5(2) N-C 2.00(1)4.8(5) 12(2) 3(1)

In Table 1, fits were done at the Zn K-edge in R-space, k^(1,2,3)weighting. 1.0<R<2.0 Å and Δk=2.275-10.61 Å⁻¹. S₀ ² was fixed at 0.95obtained by fitting the reference Zn foil. The number given in theparentheses represents the uncertainty of the last digit of the fittingresult.

The synthesized N—C zeolitic imidazolate framework contains 4.23 wt % ofN and 2.16 wt % of Zn, as determined by inductively coupled plasmaoptical emission spectrometry (ICP-OES) results shown in the “N—C” entryof Table 2.

TABLE 2 Element contents in the N-C and FeNC-CVD-T determined byICP-OES. wt% Fe Zn N C N-C — 2.16 4.23 84.00 FeNC-CVD-650 2.25 1.05 3.9785.42 FeNC-CVD-750 2.00 0.12 4.24 85.48 FeNC-CVD-900 3.76 0.23 3.3285.42 FeNC-CVD-1000 2.72 0.03 2.36 84.20

Example 2. Functional Characterization of the Catalyst

The catalyst produced according to Example 1 was characterized. As shownin FIG. 1 , the ORR (oxidization reduction reaction) performance of thecatalyst was high compared to catalysts made by conventional methods.The highest reported ORR activity of Fe—N—C catalysts pyrolyzed bymixing the Fe, N, and C precursors together is ˜22 mA·cm⁻² at 0.90 ViR-free in H₂—O₂ PEMFCs. In contrast, the Fe—N—C catalyst disclosedherein delivers a current density of 33 mA·cm⁻² at 0.90 V iR-free inH₂—O₂ PEMFCs under the same conditions. The steady-state RDEpolarization plot in FIG. 1 was obtained by using a 20 mV potential stepand 25 s potential hold time at every step in O₂-saturated 0.5 M H₂SO₄from 0.05 to 0.95 V with a rotation rate of 900 rpm at room-temperature.

In FIG. 2 , the cyclic voltammogram (CV) of the same catalyst wasacquired after the ORR polarization curve (presented in FIG. 1 ) in thesame electrolyte with a scan rate of 10 mV·s⁻¹ at room-temperature.

The H₂—O₂ PEMFC polarization curves with and without iR-correction areshown in FIG. 3 and were measured with the cathode having approximately6.0 mg·cm⁻² of the catalyst and an anode having about 0.3 mg_(Pt)·cm⁻²Pt/C. The membrane was Nafion 212, and 200·mL/min⁻¹ gas was fed at bothanode (H₂) and cathode (O₂) with 100% RH, and 1.0 bar partial pressureeach side. The cell was at 80° C., and the electrode area was 5 cm². InFIG. 4 , the Tafel plot derived from the iR-corrected ORR polarizationcurve displayed in FIG. 3 is shown to manifest the activity at 0.9 V incomparison with the DOE 2025 target (the DOE 2025 target is 44 mA·cm⁻²at 0.90 V).

For example, anhydrous FeCl₃ (99%, Sigma-Aldrich) was chosen as an Fesource because of its low boiling point, ˜316° C. The low boiling pointcan allow for generation of iron chloride vapor at low temperature(Kanari, N, et al., 2010; Rustad, D S, and Gregory, N W, 1983). Afurnace tube was configured to contain a flow of an inert gas. 80 mg ofanhydrous FeCl₃ was placed in a boat in the tube at the upstream of thegas flow. 80 mg of the N—C zeolitic imidazolate framework was placed inanother boat in the form of a thin layer downstream of the FeCl₃. Therewas a 1 cm gap between the two boats with one end cutting off. Thefurnace was heated up to a variety of temperatures with a ramping rateof 25° C. per minute, and then the temperature was held at a targettemperature (T) for three hours, followed by cooling down to roomtemperature naturally. The furnace was continuously flowed with Ar gaswith a flow rate of 0.65 L·min⁻¹ during the heating and cooling. Thepowders were then collected from the N—C zeolitic imidazolate frameworkboat. The collected powders were then subjected to magnetic purificationby slowly moving a small magnet ˜0.5 cm above to remove Fenanoparticles. The purified powders were labelled FeNC-CVD-T, where Trepresents the pyrolysis temperature in ° C. For example, Table 2 showsthe elemental content for FeNC-CVD-650, FeNC-CVD-750, FeNC-CVD-900, andFeNC-CVD-1000. The catalyst powders were stored in a vacuum desiccatorbefore being subjected to RDE and PEMFC evaluations.

Example 3: Evaluation of the ORR Activity and Performance of theFeNC-CVD-750

To prepare the catalyst powders for electrochemicalcharacterization-rotating disk electrode (RDE), the catalyst powderswere deposited on a glassy carbon working electrode. Catalyst inks wereprepared by dispersing 10 mg of the catalyst powder in a mixture ofMillipore water (36.5 μL, 18.2 MΩ cm) and ethanol (300 μL,Sigma-Aldrich, 99.8%), into which 5 wt % Nafion solution (108.5 μL,Sigma-Aldrich) was added as a binder phase. The resulting mixture wassonicated for 60 minutes in an ice bath, and then an aliquot of 8.8 μLwas drop-cast onto the glassy carbon electrode (0.247 cm², Pineinstrument), resulting in a loading of 800 μg·cm⁻². The workingelectrode with the deposited catalyst layer was used in athree-electrode cell set-up connected to a bipotentiostat (Biologic SP300) and rotator (Pine Instruments). A graphite rod and reversiblehydrogen electrode (RHE) were used as counter and reference electrodes,respectively. The ORR performance was evaluated via steady-state RDEpolarization in O₂-saturated 0.5 M H₂SO₄ using a rotation rate of 900rpm, 20-mV potential steps from 0.05 to 0.95 V, and a 25-s potentialhold time at each step at room temperature. The cyclic voltammetry (CV)was carried out between 0.05 to 0.95 V vs. RHE with a scan rate of 10mV·s⁻¹ in Ar-saturated 0.5 M H₂SO₄ electrolyte. On the other hand, theORR performance of Pt/C was evaluated in O₂-saturated 0.1 M HClO₄ usinga rotation rate of 900 rpm and a scan rate of 10 mV·s⁻¹ at roomtemperature. The ORR polarization curve was corrected by the CV obtainedby scanning the electrode between 0.05 to 0.95 V vs. RHE with a scanrate of 10 mV·s⁻¹ in Ar-saturated 0.1 M HClO₄. The hydrogenunderpotential (HUPO) charge was determined by integrating the HuPOpeaks in the potential range of 0.05-0.45 V.

The ORR activities of FeNC-CVD-650, FeNC-CVD-750, FeNC-CVD-900, andFeNC-CVD-1000 were measured using the rotating disk electrode (RDE).Among the four catalysts, FeNC-CVD-750 exhibited the highest ORRactivity (FIG. 10 ) with a half wave potential of 0.85 V (all potentialsare versus reversible hydrogen electrode) when using a catalyst loadingof 800 μg·cm⁻² and oxygen-saturated 0.5 M H₂SO₄ (FIG. 5A). FIG. 5A showsthe ORR polarization curve of the FeNC-CVD-750 catalyst (lower curve)compared to the ORR polarization curve of the N—C zeolitic imidazolateframework (N—C, upper curve). Measurement is steady-state RDEpolarization curve in room-temperature, O₂-saturated 0.5 M H₂SO₄ using arotation rate of 900 rpm, 20 mV potential steps from 0.05 to 0.95 V, anda 25 seconds potential hold time at each step. The corresponding kineticcurrent density derived from the current at 0.8 V and the limitingcurrent using the Koutecky-Levich equation is 20 mA·cm⁻² or 25 A·g⁻¹(FIG. 5B). These half-wave potential and mass activity values are amongthe highest reported for PGM-free catalysts in acidic electrolyte(Beltren, D E, and Litster, S, 2019). The kinetic current densityexhibits a Tafel slope of ˜60 mV/dec above 0.8 V (FIG. 5B), which hasbeen commonly reported for Fe—N—C catalysts (Osmieri, L, et al. 2019;Li, J, et al., 2017). This Tafel slope is comparable to that of Pt-basedcatalysts (Gasteiger, H A, et al., 2005), indicating that they share thesame rate determining step for the ORR.

An electrochemical characterization-fuel cell was prepared. TheFeNC-CVD-750 catalyst was used to prepare the cathode for MEA tests in aPEMFC under H₂—O₂ and H₂-air conditions. The cathode catalyst inks wereprepared by dispersing calculated amount of catalyst powder and NafionD521 dispersion (Ion power) into 50 wt/% 1-propanol aqueous solution for3 hours under ice bath sonication. The inks were coated layer by layeron SGL 29-BC gas diffusion layer (Sigracet) until 6 mg·cm⁻² loading wasachieved. A commercial Pt gas diffusion electrode (0.3 mg_(Pt)cm⁻², fuelcell store) was used as the anode. The anode electrode was first hotpressed onto NR212 membrane (Ion Power) at 130° C. for 4 minutes. Beforehot pressing the cathode on the opposite side of the membrane, a thinNafion overspray layer was applied on the cathode catalyst layer toreduce the interfacial resistance. The cathode was then hot pressed onthe previously-pressed half MEA at 130° C. for 2 minutes. The MEA wasthen assembled into a single cell with single-serpentine flow channels.The single cell was then evaluated in a fuel cell test station (100 W,Scribner 850e, Scribner Associates). The cells were conditioned under N₂environment, at 100% relative humidity and 80° C. for at 2 hours. Oxygenflowing at 1000 mL·min⁻¹ and H₂ (purity 99.999%) flowing at 200 mL·min⁻¹were used as the cathode and anode reactants, respectively. The backpressures during the fuel cell tests are 1.0 bar reactant gas. The vaporpressure is around 0.5 bar owing to the 100% relative humidity. Thus,the total pressure applied to the MEA is around 1.5 bar (150 KPa). Fuelcell polarization curves were recorded in a voltage control mode.

The H₂-air performance of FeNC-CVD-750 was evaluated in a differentialcell owing to its superior mass transport of air. The protocol isotherwise largely the same as that applied for the H₂—O₂ PEMFCevaluation. Few differences include (1) the cathode FeNC-CVD-750catalyst loading was 4 mg·cm⁻²; (2) the anode loading was 0.2mg_(Pt)cm⁻²; and (3) air flowing at 1000 mL·min⁻¹ and H₂ (purity99.999%) flowing at 200 mL·min⁻¹ were used as the cathode and anodereactants, respectively.

The FeNC-CVD-750-containing electrode was evaluated in an H₂—O₂ PEMFCfor three full polarization scans, followed by acquisition of H₂-airpolarization curves on the same membrane electrode assembly (MEA). Acurrent density of 44 mA·cm⁻² is reached at 0.89 V_(iR-free) during thefirst scan (increasing-current) in the H₂—O₂ PEMFC at 1.0 bar partialpressure of O₂ and 80° C., 0.01 V lower than the DOE 2025 target (FIG. 3and FIG. 4 ). In FIG. 3 , the Cathode includes: ˜6.0 mg·cm⁻² of theFeNC-CVD-750 catalyst; the anode: 0.3 mg_(Pt)cm⁻² Pt/C; membrane: Nafion212; 200 mL·min⁻¹ gas fed at the anode (H₂) and 1000 mL·min⁻¹ at thecathode (O₂) at 100% RH, 1.0 bar partial pressure H₂ and O₂, 80° C.,electrode area 5 cm². A current density of 33 mA·cm⁻² was achieved at0.9 V_(iR-free) and 380 mA·cm⁻² at 0.8 V_(iR-free), both exceeding thoseof all previous PGM-free catalysts reported to date in H₂—O₂ PEMFCsunder similar conditions (FIG. 5C).

The current at 0.9 V_(iR-free) drops to 22 mA·cm⁻² and then 18 mA·cm⁻²on the second and third scans, respectively (FIG. 11A and FIG. 11B)indicating that the FeNC-CVD-750 catalyst has poor stability in H₂—O₂PEMFCs, similar to all highly active Fe—N—C catalysts (Shao, Y, et al.,2019; Osmieri, L, et al., 2020). This result is expected since all theseFe—N—C catalysts, including FeNC-CVD-750, likely share the same Fe—N₄active sites. Despite the degradation, a maximum power density of 0.37W·cm⁻² was obtained in the subsequent H₂-air PEMFC testing (FIG. 5D),which is among the highest values reported for PGM-free catalysts thusfar. The conditions for FIGS. 11A and 11B were cathode: ˜6.0 mg·cm⁻² ofthe FeNC-CVD-750 catalyst; anode: 0.3 mg_(Pt)cm⁻² Pt/C; Membrane: Nafion212; 200·mL/min⁻¹ gas fed at both anode (H₂) and cathode (O₂) with 100%RH, and 1.0 bar partial pressure each side; cell 80° C.; electrode area5 cm².

Example 4: Characterization of the FeNC-CVD-750

To understand the source of its exceptional ORR activity, theFeNC-CVD-750 was characterized using multiple techniques. Arepresentative SEM image of FeNC-CVD-750 (FIG. 6A) shows a similarpowder morphology to the N—C zeolitic imidazolate framework (FIG. 2F),without noticeable particle growth and aggregation. Meanwhile, the ADF-and secondary electron (SE)-STEM images, at different magnifications,show the highly porous morphology of the carbon matrix and absence ofmetal clusters (FIG. 6B-6D). Moreover, the XRD pattern (FIG. 12A), the Cand N contents (Table 2), and the XPS Nis spectrum (FIG. 6E and FIG.12B) of FeNC-CVD-750 are nearly the same as those of the N—C zeoliticimidazolate framework. These results show that the overall morphology ofthe N—C zeolitic imidazolate framework is largely preserved after theCVD at 750° C., which is expected considering that the N—C zeoliticimidazolate framework was synthesized using pyrolysis at 1050° C. priorto the CVD of Fe. On the other hand, the Zn content drops from 2.16 wt %in the N—C zeolitic imidazolate framework to 0.12 wt % in FeNC-CVD-750,accompanied by incorporation of 2.00 wt % Fe (see Table 2). The presenceof abundant Fe—N_(x) moieties in FeNC-CVD-750 is directly evidenced byatomic resolution ADF-STEM imaging coupled with electron energy lossspectroscopy (EELS). Abundant bright dots are clearly seen in theADF-STEM image (FIG. 6G, inset), for which the EELS point spectrum showsthe close proximity of single Fe atom and N (FIG. 6G, FIG. 13A, and FIG.13B).

The presence of Fe—N₄ moieties in FeNC-CVD-750 is also supported by the⁵⁷Fe Mössbauer spectrum collected at 5 Kelvin (FIG. 6H). The lowestpossible temperature during Mössbauer data acquisition is important todistinguish superparamagnetic Fe species (such as nano-Fe-oxides) fromFe—N₄ sites. While both nano-Fe-oxides and O₂—Fe(III)-N₄ sites lead to asimilar D1 signal at room temperature, this degeneracy is usuallyunveiled at 5 K: nanosized superparamagnetic Fe oxides convert into asextet component while O₂—Fe(III)-N₄ sites still contribute with a D1component (Li, J, Jia, Q, et al., 2019).

The ⁵⁷Fe Mössbauer spectrum at 5K of FeNC-CVD-750 identifies twodoublets, labelled D1 and D3, representing 89% and 11% of the absorptionarea, respectively (FIG. 6H and Table 3).

TABLE 3 Parameters obtained from fitting the Mdssbauer s pectrum ofFeNC-CVD-750 acquired at 5 K (Fig. 6H): relative area (RA, %), isomershift (IS, mm s⁻¹), quadrupole splitting (QS, mm s⁻¹), and line width(LW, mm s⁻¹) of each component. RA IS QS LW Comp. % mm s⁻¹ mm s⁻¹ mm s-1Assignment Doublet 1 89 0.50 1.02 1.25 σ² − Fe(lll)-N₄ Doublet 3 11 0.973.56 0.92 FeCI₂ · xH₂O

D1 has been commonly observed for Fe—N—C materials and has recently beenassigned to O₂—Fe(III)-N₄ (Li, J, Ghoshal, S, et al., 2016; Li, J, Jiao,L, et al., 2020; Mineva, T, et al., 2019). D3 can be unambiguouslyassigned to a high spin Fe²⁺ species, due to its high IS of 0.97 mm/s. Adoublet with similar isomer shift (IS) and quadrupole slitting (QS) thanthose of D3 here were previously observed in the Fe—N—C catalystssynthesized via low temperature imprinting iron chlorides into N-dopedcarbon matrix at both room temperature and 4.2 K (Menga, D. et al.Active-Site Imprinting: Preparation of Fe—N—C Catalysts from ZincIon-Templated Ionothermal Nitrogen-Doped Carbons. Advanced EnergyMaterials 9, 1902412 (2019)). Meanwhile, the Mössbauer spectrum of thepowdered FeCl₂.4H₂O crystal collected at 4.2 K exhibits a QS (3.13 mm/s)close to that of the D3 (3.56 mm/s) observed here, but a higher IS of1.47 mm/s (Ono, K., Shinohara, M., Ito, A., Fujita, T. & Ishigaki, A.Mössbauer Study of FeCl₂.4H2O in the Temperature Range 4.2° to 0.025° K.J. Appl. Phys. 39, 1126-1127 (1968)). Based on these results, plus theabsence of the FeCl₂.4H₂O signal in the XRD pattern of FeNC-CVD-750(FIG. 12A), it is tentatively assigned, the D3 component in FeNC-CVD-750to amorphous FeCl₂.xH₂O. This assignment quantitatively agree with theXPS results of ˜90% Fe(III) and ˜10% Fe(II), and together leads toU_(Fe)=˜90% and U_(FeN4)=100%, and an overall high Fe utilization (U) of˜90%, according to Equation 2.

To further prove this, the electrochemically active Fe content in theFeNC-CVD-750 RDE electrode was determined from the area of the redoxpeak around 0.66 V, shown in FIG. 7A. These redox peaks, which areabsent in the CV of the N—C zeolitic imidazolate framework (N—C trace inFIG. 7A does not have redox peaks), have been previously assigned to theFe(III)/Fe(II) redox transition of Fe—N₄ sites in H₂SO₄ solution (Wu, G,et al., 2011). The electroactive Fe content on the RDE electrode wasfound to be 14.7 μg·cm⁻². Given the catalyst loading of 800 μg·cm⁻² andan Fe content of 2.0 wt % for FeNC-CVD-750 (Table 2), the overallutilization (U) of FeNC-CVD-750 is found to be 92% (Equation 6). Thisvalue agrees well with the U of ˜90% derived from spectroscopic analysisand the aforementioned assignments of D1 to air-accessible Fe—N₄ sitesand D3 to FeCl₂, thereby confirming the full utilization of Fe—N₄ sitesin FeNC-CVD-750 (U_(FeN4)=100%).

To further confirm that all the Fe—N₄ sites in FeNC-CVD-750 areelectrochemically active during the ORR, in situ XAS was conducted onFeNC-CVD-750 at the Fe K-edge in an O₂-purged 0.5 M H₂SO₄ electrolyte ina flow cell as a function of applied potential. The XANES and FT-EXAFSspectra collected at 0.9 V nearly overlap that of Fe(III)Pc-O₂ (FIG.7B), which confirms that the vast majority of Fe—N₄ sites are in theform of Fe(III)-N₄—O₂ at 0.9 V (Table 4).

TABLE 4 Summary of the fitting results of the FT-EXAFS spectra collectedat the K-edge of the FeNC-CVD-750 ex situ exposed to air or at 0.9 V in02-saturated 0.5 M H2SO4. σ² ×10⁻³ R(Å) N (Å2) E₀ (eV) Ex 2.02(1) 5.9(9)6(2) 0.7(1.0) situ 0.9 V 2.00(2) 6 (1) 6(4) −1(1)

In Table, 4, fits were done at the Fe K-edge in R-space, k^(1,2,3)weighting. 1.0<R<2.1 Å and Δk=2.3-11.0 Å⁻¹ were used for fitting. S₀ ²was fixed at 0.81 obtained by fitting the reference foil. The numbergiven in the parentheses represents the uncertainty of the last digit ofthe fitting result.

As the potential is gradually reduced to 0.5 V, the XANES spectrumshifts negatively, and correspondingly the intensity of the FT-EXAFSpeak drops (FIG. 7C). These occurrences have been commonly observed onFe—N—C catalysts and ascribed to the redox transition from Fe(III)-N₄—O₂to Fe(II)-N₄ (Zitolo, A, et al., 2017, Li, J, et al., 2016, Osmieri, L,et al., 2019, Li, J, et al., 2017). However, the FT-EXAFS spectrum at0.5 V exhibits a shoulder around 1.8 Å, rather than just the oneprominent peak observed at 0.9 V (FIG. 7C), and it cannot be fitted withan Fe—N₄ model. A recent in situ Mössbauer and XAS study in H₂SO₄solution proposed that as the potential is decreased, the Fe(III)-N₄—O₂moiety is converted to Fe(II)-N₄ with an axial bond with a sulfate ion,Fe(II)—N₄—SO₄ (Zelenay, P, Myers, D J, 2020). Accordingly, the Fespecies at 0.5 V was assigned to Fe(II)-N₄—SO₄. The conversion of one Fespecies to another with changing potential applied to FeNC-CVD-750 isfurther confirmed by the existence of isobestic points at 7132 and 7154eV in the XANES spectra (FIG. 7B). By taking the XANES spectra at 0.9 Vand 0.5 V as standards representing Fe(III)-N₄—O₂ and Fe(II)—N₄—SO₄,respectively, the fraction of Fe(III)(ΘFe(III)(E)=Fe(III)/(Fe(III)+Fe(II)); E represents the appliedpotential) can be acquired by linear combination analysis (LCA) of thein situ XANES spectra. As seen in FIG. 7D, the ΘFe(III)(E) acquired bythe LCA closely follows the one calculated from Equation 3 (Gottesfeld,2014):

$\begin{matrix}{{\Theta_{{Fe}({III})}(E)} = \frac{1}{1 + e^{\frac{- F}{RT}{({E - E_{redox}})}}}} & {{Equation}3}\end{matrix}$

In Equation 3, F is the Faraday constant, R is the universal gasconstant, T is the temperature, and Eredox is the Fe(II)/Fe(II) redoxpotential derived from the redox peaks in the CV of FeNC-CVD-750 indeaerated electrolyte (FIG. 7A), 0.66 V. These in situ XAS results thuscorroborate full utilization of Fe—N₄ sites in FeNC-CVD-750 during theORR.

The simultaneous achievements of U_(FeN4)=100% and U_(Fe)=˜90% leads toan ultrahigh U of ˜90% on FeNC-CVD-750 that meanwhile possesses arelatively high Fe content of 2 wt %. This U is six times higher thanthat (14.1%) of a state-of-the-art Fe—N—C catalyst with comparable bulkFe content (2.14 wt %) (Wan, X, et al., 2019). This indicates that thenegative correlation between the Fe wt % and U in traditional Fe—N—Ccatalysts has been substantially alleviated in FeNC-CVD-750 using theCVD method. Consequently, the FeNC-CVD-750 catalyst has a record-highSD_(mass) of 2×10²⁰ sites·g⁻¹ (Equation 5), which is more than threetimes higher than the values reported thus far for Fe—N—C catalysts(Wan, X, et al., 2019, Primbs, M, et al., 2020, Luo, F, et al., 2019)and approaching the SD_(mass) of 47% Pt/C (3.2×10²⁰ sites·g⁻¹)(Gasteiger, H A, et al., 2005).

For calculations of the SD_(mass) and TOFs of FeNC-CVD-750, theelectrochemically active Fe—N₄ site density (S.D.) on the RDE electrodeof FeNC-CVD-750 is calculated from the electric charge derived from thearea of the FeII/FeIII redox peak (Aredox=2.55×10⁻⁴ A·V·cm⁻²) in the CVpresent in FIG. 4A by the following equation:

$\begin{matrix}{{S.D.} = {\frac{{A_{redox}\left\lbrack {A \cdot V \cdot {cm}^{- 2}} \right\rbrack} \times {N_{A}\left\lbrack {{atom} \cdot {mol}^{- 1}} \right\rbrack}}{{W\left\lbrack {V \cdot s^{- 1}} \right\rbrack} \times {F\left\lbrack {s \cdot A \cdot {mol}^{- 1}} \right\rbrack}} = {{1.6} \times 10^{17}\left( {{sites} \cdot {cm}^{- 2}} \right)}}} & {{Equation}4}\end{matrix}$

assuming one active Fe—N₄ site transfer one electron during theFe(II)/Fe(II) redox transition. N_(A) IS Avogadro's constant; F isFaraday constant; and W is the scan rate of the CV (FIG. 3D) (0.01V·s⁻¹).The SD_(mass) of FeNC-CVD-750 is determined by dividing the S.D. by themass of the FeNC-CVD-750 catalyst on the RDE electrode per electrodearea (L=800 μg·cm⁻² or 8×10⁻⁴ g·cm⁻²):

$\begin{matrix}{{SD_{mass}} = {\frac{S.{D\left\lbrack {{sites} \cdot {cm}^{- 2}} \right\rbrack}}{L\left\lbrack {g \cdot {cm}^{- 2}} \right\rbrack} = {2 \times 10^{20}\left( {{sites} \cdot g^{- 1}} \right)}}} & {{Equation}5}\end{matrix}$

The Fe utilization (U_(Fe)) is in turn derived from the S.D. and the Feloading on the RDE electrode with the following equation:

$\begin{matrix}{U_{Fe} = {\frac{{S.{D\left\lbrack {{sites} \cdot {cm}^{- 2}} \right\rbrack}} \times {M_{Fe}\left\lbrack {g \cdot {mol}^{- 1}} \right\rbrack}}{{L\left\lbrack {g \cdot {cm}^{- 2}} \right\rbrack} \times {{Fe}{wt}}\% \times {N_{A}\left\lbrack {{atom} \cdot {mol}^{- 1}} \right\rbrack}} = {92\%}}} & {{Equation}6}\end{matrix}$

Wherein M_(Fe) is the molar mass of iron; and the Fe wt % is 2 wt F_(e)%.The TOF (e⁻·site⁻¹s⁻¹, at 0.8 V) is calculated from the ik (25 A·g⁻¹) at0.8 V and SD_(mass) based on the following equation:

$\begin{matrix}{{TOF} = {\frac{{i_{k}@0.8}{V\left\lbrack {A \cdot g^{- 1}} \right\rbrack} \times {N_{A}\left\lbrack {{site} \cdot {mol}^{- 1}} \right\rbrack}}{{{SD}_{mass}\left\lbrack {{sites} \cdot g^{- 1}} \right\rbrack} \times {F\left\lbrack {s \cdot A \cdot {mol}^{- 1}} \right\rbrack}} = {{0.7}8\left( {e^{-} \cdot {site}^{- 1} \cdot s^{- 1}} \right)}}} & {{Equation}7}\end{matrix}$

The TOF of the Fe—N₄ sites in FeNC-CVD-750 at 0.8 V is 0.78e⁻·site⁻¹·s⁻¹, as derived from the kinetic current density at 0.8 V andSD_(mass) (Equation 7). This value is comparable to those of previousFe—N—C catalysts (Zelenay, P, and Myers, D J, 2020; Kramm, U I, et al.,2012), but one order of magnitude lower than that of Pt/C (Paulus, U A,et al., 2002, Gasteiger, et al., 2005). Therefore, the ultra-highkinetic current density of FeNC-CVD-750 is mainly ascribed to therecord-high SD_(mass) achieved by CVD, not to a record-high TOF comparedto previous state-of-art Fe—N—C catalysts.

The high SD_(mass) of FeNC-CVD-750 is necessary for its high performancein an MEA. In addition, the ADF-STEM images of the cathode of the MEAshow the preservation of the particle morphology of FeNC-CVD-750 withoutnoticeable agglomeration (FIG. 8A and FIG. 14A). The correspondingoverlaid EDS images show relatively uniform distribution of Fe atoms inthe electrode and fluorine from the ionomer distributed over the surfaceof the carbon particles, suggesting these two elements are located inclose proximity to one another (FIG. 14B, FIG. 14C, FIG. 14D). Theseresults indicate that the Fe—N₄ sites in the cathode are readilyaccessible to protons. The preservation of the particle morphology ofFeNC-CVD-750 in an MEA and the high accessibility of Fe—N₄ sites toprotons and O₂ indicate that its high SD_(mass) can be utilizedefficiently, accounting for the exceptional ORR performance ofFeNC-CVD-750 in a PEMFC, at both low and high current densities.

Example 5: The Mechanism of the Formation of Fe—N₄ Sites by CVD

Next, it was shown that the CVD approach leads to a different Fe—N₄ siteformation mechanism from the previous synthetic approaches. Thebackground CV of FeNC-CVD-750 is much broader than that of N—C zeoliticimidazolate framework, in addition to the emergence of theFe(III)/Fe(II) redox peaks (FIG. 7A). The double-layer capacitance ofthe N—C zeolitic imidazolate framework derived from the CV at 0.3 V is˜0.16 F·mg⁻¹. Assuming a specific capacitance of the carbon surface of204 mF·m⁻² (Ghoshal, et al., 2016), this corresponds to anelectrochemical surface area (ECSA) of ˜812 m²·g⁻¹. This value matchesits Brunauer-Emmett-Teller area of 807 m²·g⁻¹. After the CVD at 750° C.,the double-layer capacitance markedly increases to ˜0.36 F·mg⁻¹,corresponding to a high ECSA of ˜1800 m²·g⁻¹, close to theBrunauer-Emmett-Teller area of ˜1593 m²·g⁻¹ of FeNC-CVD-750. Thedifferential pore distribution analysis shows a substantial increase inthe abundance of both micropores (<2 nm) and mesopores (FIG. 15A) afterthe CVD.

The dramatic enhancement of the ECSA of the N—C zeolitic imidazolateframework after the CVD at 750° C. does not occur when the CVD isperformed at 650° C. The CV of FeNC-CVD-650 is only slightly broaderthan that of the N—C zeolitic imidazolate framework (FIG. 15B). Inaddition, the SD_(mass) of FeNC-CVD-650 derived from the redox peak area(FIG. 15B) is 5×10¹⁹ sites·g⁻¹, much lower than that of FeNC-CVD-750.Meanwhile, the Zn content in FeNC-CVD-650 remains high at ˜1.05 wt %(Table 2), much higher than that (0.12 wt %) observed in FeNC-CVD-750.This comparison suggests that the enhancement of the porosity of N—Czeolitic imidazolate framework and the formation of Fe—N₄ sites areclosely related to the removal of Zn. The substantial loss of Zn uponCVD at 750° C. is not caused by the evaporation of metallic Zn since750° C. is much lower than the boiling point of Zn (907° C.). Znevaporation started only above 850° C. in previous syntheses ofZIF-8-derived Fe—N—C catalysts (Zhang, H, et al., 2017, Proietti, E, etal., 2011). It was noted that (1) ZnCl₂ has a lower boiling point of732° C., (2) Zn is nearly completely removed after CVD at 750° C. butnot at 650° C., (3) the removal of 2.16 wt % Zn is accompanied by anincrease of Fe content by a comparable amount, 2.00 wt % (Table 2),without changing the metal-N XPS peak significantly (FIG. 12B), and (4)both the Zn in the N—C zeolitic imidazolate framework and the Fe in theFeNC-CVD-750 are in the form of Me-N₄. These combined results lead tothe proposal that the Fe—N₄ sites are formed via the followingDisplacement Reaction 3 during the CVD at 750° C. (FIG. 1A):

2FeCl₃ (g)+2Zn—N₄→2Fe—N₄+2ZnCl₂ (g)+Cl₂ (g)   Reaction 3

According to this Fe—N₄ formation mechanism, the penetration of the ironchloride vapor into the N—C zeolitic imidazolate framework plus releaseof ZnCl₂ vapor are responsible for the dramatic enhancement in porosityor ECSA upon the CVD at 750° C. This reaction also rationalizes theoptimized temperature of 750° C. for the CVD method. A temperature of750° C. is marginally higher than the boiling point of ZnCl₂ (732° C.),so the ZnCl₂ is readily released in the vapor form, thereby promotingthe displacement reaction (Reaction 3), forming Fe—N₄.

Meanwhile, The Fe—N₄ sites are more thermally stable at 750° C. than athigher temperature. This is reflected by the observation that the Ncontent in FeNC-CVD-750 is highly comparable to that of the N—C zeoliticimidazolate framework, but drops precipitously at higher temperatures(Table S2). The rapid drop of the N content in Fe—N—C catalysts withincreasing pyrolysis temperature has been commonly observed and regardedas one key factor limiting the Fe—N₄ site density (Zhang, H, et al.,2017, Proietti, E, et al., 2011). Therefore, the Fe—N₄ sites are betterpreserved in the synthesis of FeNC-CVD-750 at a temperatureapproximately 200° C. lower than that (1000±100° C.) utilized for thesynthesis of previous state-of-the-art Fe—N—C catalysts.

The displacement reaction (Reaction 3) also accounts for the fullutilization of Fe—N₄ sites in FeNC-CVD-750. With this mechanism, theFe—N₄ sites are formed at the locations where the Zn—N₄ sites areaccessible by iron chloride vapor, and thus accessible by air. Inaddition, the release of ZnCl₂ vapor from the formed Fe—N₄ sites mayfurther improve their accessibility. In contrast, previous synthesismethods extensively mix Fe, N, and C precursors prior to pyrolysis (FIG.1B). Consequently, the Fe—N₄ moieties are distributed throughout thecarbon matrix, whereas only those in the outer-surface region areaccessible by air. These Fe—N—C catalysts show both the D1 and D2spectroscopic signatures in their ⁵⁷Fe Mössbauer spectra, in comparableamounts, with D2 recently assigned to Fe(II)-N₄ moieties free ofadsorbed oxygenated species atop the ferrous central cation (Mineva, T,et al. 2019). These assignments strongly suggest that the D2 signaturecorresponds to Fe—N₄ sites located in the bulk. For example, D2accounted for 49% and 62% of the relative absorption area of Mössbauerspectra of the ICL and UNM catalysts, the two benchmark Fe—N—C catalystswith the highest U_(FeN4) values (Primbs, M, et al., 2020). Hence, thecurrent state-of-art Fe—N—C catalysts have a U_(FeN4) significantlylower than 100%. Therefore, although full utilization of active siteshas long been conceived as a unique advantage of single-atom catalysts,this is the first time it has been realized in Fe—N—C catalysts byresorting to a dual-step synthesis and CVD deposition of Fe.

Collectively, CVD (at 750° C.) possesses two essential advantagescompared to previous synthesis approaches for Fe—N—C catalysts: (1) theFe—N₄ sites are formed at a much lower temperature (allowing increasedN-content and therefore increased Fe—N₄ sites, as well as mitigated Feclustering) and (2) the Fe—N₄ sites are located on the outer-surface ofthe material with full site utilization (U_(FeN4)=100%). Consequently,the FeNC-CVD-750 catalyst possesses a record-high SD_(mass) and ORRperformance in H₂—O₂ PEMFCs. It is also a model catalyst containing onlyone type of active site. Therefore, model catalyst and practicalcatalyst for the ORR are combined in a single entity by using the CVDapproach. The CVD approach pioneered here is widely applicable to thesynthesis of single-atom catalysts with other metals (Mn, Co) and othersubstrates (metal oxides) for many applications.

Physical Characterizations

Inductively coupled plasma optical emission spectrometry (ICP-OES): TheICP-OES tests were conducted at Robertson Microlit Laboratories.

TEM: Transmission electron microscope (TEM) image of the ZIF-8 wasconducted on a JEOL 2010 field emission gun (FEG).

STEM: Aberration-corrected scanning transmission electron microscopy(AC-STEM) was conducted using a JEOL NEOARM TEM/STEM operated at 80 keVand equipped with a Gatan Quantum electron energy loss spectrometer anddual 100 m² silicon drift detectors for energy dispersive X-rayspectroscopy.

SEM: Scanning electron microscopy (SEM) micrographs of N—C zeoliticimidazolate framework were obtained with a Hitachi S-4800 apparatus(Hitachi, Tokyo, Japan).

XRD: X-ray diffraction (XRD) patterns were conducted using aPANanalytical X'Pert Pro powder X-ray diffractometer with Cu K_(α)radiation.

XPS: X-ray photoelectron spectroscopy (XPS) tests were done with KratosAXIS Ultra DLD spectrometer with Al Kα (1486.6 eV) X-ray source at UCLA.

N₂ adsorption/desorption analysis: N₂ sorption analysis was performed atliquid nitrogen temperature (77 K) with a Micromeritics ASAP 2020instrument. Prior to the measurements, all samples were degassed at 200°C. for 5 h in flowing nitrogen to remove guest molecules or moisture.The pore size distributions were calculated by fitting the full isothermwith the quench solid density functional theory model with slit poregeometry from NovaWin (Quantachrome Instruments).

Mössbauer spectroscopy: ⁵⁷Fe Mössbauer spectroscopy was used to obtaininformation on iron speciation. Samples of ˜300 mg were mounted in a 2cm² holder. Mössbauer spectra were measured at 5 K in a helium flowcryostat (SHI-850 Series from Janis, USA). The Mössbauer spectrometer(Wissel, Germany) was operated in the transmission mode with a ⁵⁷Co: Rhsource at room temperature. The velocity driver was operated in theconstant acceleration mode with a triangular velocity waveform. Thevelocity scale was calibrated with the magnetically split sextet of ahigh-purity α-Fe foil at room temperature. The spectra were fitted toappropriate combinations of Lorentzian profiles representing quadrupoledoublets and sextets by least-squares methods. Isomer shifts are givenrelative to α-Fe at room temperature.

XAS measurements. The ex situ XAS measurements at the Zn K-edge ofZn(II)Pc, ZIF-8, and N—C zeolitic imidazolate framework were performedin transmission mode at beamline 10-ID of the Materials ResearchCollaborative Access Team (MRCAT) at the Advanced Photon Source, ArgonneNational Laboratory, Lemont, Ill., United States. Ex situ XASmeasurements at the Fe K-edge of Fe-based catalysts were conducted atbeamline ISS 6-EM and 8-ID in fluorescence mode in National SynchrotronLight Source II (NSLS-II) (Brookhaven National Laboratory, NY). Inaddition, in situ XAS measurements were conducted on FeNC-CVD-750. Theink for the XAS electrode was composed of 1:3 (wt %) 18.2 MO puritydeionized water (Millipore) and 2-propanol (HPLC-grade, Aldrich), a 5 wt% Nafion solution (Aldrich), and FeNC-CVD-750 catalyst powder. The inkswere directly sprayed onto a Zoltek® carbon cloth on a piece of heatedglass. The final Fe loading is ˜0.05 mgFe·cm-2 in the electrodes (1×3cm²). Ex situ XAS data were firstly collected on the dry electrode,which was then conditioned in 0.5 M H₂SO₄ under vacuum for three hoursto remove the oxides, impurities, and gases trapped inside theelectrode, and to thoroughly wet the electrodes. Afterwards, theelectrode was mounted onto a electrochemical half-cell reportedpreviously (Newville, 2001) and further conditioned electrochemicallyfor 50 cycles between 0.05 and 0.95 V with a scan rate of 50 mV s⁻¹ inN₂-saturated 0.5 M H₂SO₄ electrolyte. Full range Fe K-edge spectra weretaken at various static potentials along the anodic sweep of the cyclicvoltammetry (CV) in O₂-saturated 0.5 M H₂SO₄ electrolyte. Data werecollected in fluorescence mode with a Fe reference foil positionedbetween 12 and 13 as a reference. The voltage cycling limits were 0.50to 0.95 V vs. RHE. The XAS data were processed and fitted using theIfeffit-based Athena and Artemis programs (Ankudinov, A L, et al.,1998). Scans were calibrated, aligned, and normalized with backgroundremoved using the IFEFFIT suite (47). The X(R) were modeled using singlescattering paths calculated by FEFF6 (from the IFEFFIT interactive XAFSAnalysis suite, Newville, 2001).

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1. An Fe—N—C catalyst comprising N—C sites and Fe atoms; wherein atleast 90% of the Fe atoms in the Fe—N—C catalyst are in Fe—N₄ moieties;and wherein the ratio of Fe—N₄ moieties located at an exterior surfaceof the Fe—N—C catalyst to the Fe—N₄ moieties located within the Fe—N—Ccatalyst is about 100:1.
 2. The Fe—N—C catalyst of claim 1, wherein theFe—N—C catalyst comprises not less than about 2 weight % of Fe relativeto the total weight of the Fe—N—C catalyst.
 3. The Fe—N—C catalyst ofclaim 1, wherein at least about 99% of the N—C sites at the exteriorsurface of the Fe—N—C catalyst are bound to Fe.
 4. The Fe—N—C catalystof claim 1, wherein at least about 99% of the Fe—N₄ moieties areaccessible by a gas-phase contacting the catalyst.
 5. The Fe—N—Ccatalyst of claim 1, wherein the electrochemical surface area of theFe—N—C catalyst is not less than about 1800 m²/g.
 6. The Fe—N—C catalystof claim 1, wherein the catalyst is capable of providing an IR-correctedcurrent ≥33 mA/cm² at 0.90 V when used in a proton exchange membranefuel cell.
 7. A cathode for a fuel cell comprising the catalyst ofclaim
 1. 8. The cathode of claim 7, wherein the fuel cell is a protonexchange membrane fuel cell.
 9. The cathode of claim 8, wherein theproton exchange membrane fuel cell is capable of an oxygen reductionreaction activity of ≥44 mA/cm² at 0.89 V_(IR-corrected).
 10. A methodof making an Fe—N—C catalyst, the method comprising: (a) providing anN-doped carbon substrate comprising a metal M in M-N₄ moieties; (b)contacting the N-doped carbon substrate with a vapor comprising FeCl₃,whereby Fe—N₄ moieties form on the N-doped carbon substrate and a vaporcomprising the metal M is released from the N-doped carbon substrate.11. The method of claim 10, wherein M is Zn.
 12. The method of claim 10,wherein the contacting is at a temperature in the range from about 600°C. to about 900° C.
 13. The method of claim 12, wherein the temperatureis about 750° C.
 14. The method of claim 10, wherein the contacting isfor about 3 hours.
 15. The method of claim 10, wherein step (b)comprises pyrolyzing the N-doped carbon substrate and a materialcomprising FeCl₃ such that at least a portion of the FeCl₃ vaporizes toa vapor comprising FeCl₃, whereby the vapor contacts the N-doped carbonsubstrate and Fe—N₄ sites form on the N-doped carbon substrate.
 16. Themethod of claim 10, wherein the vapor comprising FeCl₃ is provided byvaporizing anhydrous FeCl₃ in a furnace.
 17. The method of claim 10wherein the vapor comprising FeCl₃ is carried with an inert gas.
 18. Themethod of claim 16, wherein the vaporizing comprises placing a materialcomprising FeCl₃ in an inert gas flow upstream of the N-doped carbonsubstrate.
 19. The method of claim 10, wherein M is Zn and a Zn-halidevapor is released from the N-doped carbon substrate during the formationof the Fe—N₄ sites.
 20. The method of claim 19, wherein the Zn-halidevapor is ZnCl₂ vapor.
 21. The method of claim 10, wherein a halide gasis released from the N-doped carbon substrate during the formation ofthe Fe—N₄ sites.
 22. The method of claim 21, wherein the halide gas ischlorine gas.
 23. The method of claim 10, further comprising purifyingthe Fe—N—C catalyst.
 24. The method of claim 23, wherein the purifyingis performed by a method comprising removal of Fe with a magnet.
 25. Themethod of claim 10, wherein the N-doped carbon substrate is prepared bya method comprising: mixing Zn(NO₃) and 2-methylimidazole in a methanolsolution until a suspension comprising a zeolitic imidazolate frameworkeight forms; isolating the zeolitic imidazolate framework eight; andoptionally pyrolyzing the zeolitic imidazolate framework eight.
 26. Themethod of claim 10, wherein the N-doped carbon substrate is prepared bya method comprising: mixing a zeolitic imidazolate framework eight with1,10 phenanthroline in a solution of ethanol and water to form a solidsuspension; and pyrolyzing the dried solid suspension under an inertgas.
 27. The method of claim 25, wherein the N-doped carbon substrate isprepared by pyrolyzing under an inert gas at about 1050° C. for aboutone hour.
 28. The method of claim 26, wherein the N-doped carbonsubstrate has a Brunauer-Emmett-Teller area of at least about 800 m²/g.